1. Field of the Invention
This invention relates to an exhaust gas purifier for purifying the exhaust gas emitted from a diesel engine to effectively crack oxides of nitrogen (NOx), for thereby discharging clean waste gas.
2. Description of the Related Art
If fuel were theoretically completely burned, an exhaust gas emitted from a vehicle engine should contain only CO.sub.2 (carbon dioxide), H.sub.2 O (water) and N (nitrogen). However, since complete combustion of the fuel is actually unattainable, the exhaust gas usually contains CO (carbon monoxide), HC (hydrocarbon) and NOx (oxides of nitrogen) as well.
Oxide in the air is essential to burn fuel gas in the engine. Approximately a quarter of the air consists of oxide, while most of the remaining three quarters are nitrogen, and minute amounts of other components. Generally, the nitrogen and oxide exist independently and are not bonded to each other in the air. However when fuel gas is burned at a high temperature, the nitrogen is oxidized, and oxides of nitrogen NOx are formed as a by-product.
A gasoline engine for an ordinary motor vehicle has a three-way catalytic converter in its exhaust system. The three-way catalytic converter not only oxides CO and HC but also reduces NOx. For this purpose, the concentration of O.sub.2 in the exhaust gas should be always kept as small as possible. When a carburetor or an electronically controlled fuel injection system with an air-to-fuel ratio control function is employed, it is necessary to control the concentration of O.sub.2 to a stoichiometric ratio based on the air-to-fuel ratio feedback control by using an O.sub.2 sensor. With the gasoline engine, the exhaust gas produced by the three-way catalytic converter includes CO, Hc and NOx and is discharged as a highly purified gas.
With a diesel engine widely used for a large motor vehicle such a bus or a truck, the three-way catalytic converter is not effective. The diesel engine is characterized in that air necessary for combustion is always supplied to the engine without controlling the amount thereof and that only the amount of the fuel is controlled. Specifically while the diesel engine is under a partial load, the fuel is burned with excessive air. Therefore, the oxide concentration in the exhaust gas is higher than the oxide concentration in the exhaust gas from the gasoline engine. A gas oil as a diesel engine fuel contains more S (sulfur) than the gasoline.
Generally speaking, the exhaust gas emitted from the diesel engine tends to have a CO concentration of 0.3% or less and 500 to 2000 ppm, and a relatively low HC concentration due to C.sub.1 to C.sub.3 and C.sub.8 contained in the fuel. However, the NOx concentration is usually above 200 ppm, which is nearly equivalent to the NOx concentration of the exhaust gas of the gasoline engine. Specifically, a direct injection type diesel engine tends to show a higher NOx concentration.
Therefore, to decrease NOx, it is not advantageous to use the conventional three-way catalytic converter to the diesel engine without any modification. Further, the exhaust gas from the diesel engine usually contains a lot of smoke mainly consisting of carbon particulates. The three-way catalytic converter cannot effectively decrease the smoke. A variety of efforts have been made to decrease NOx and the smoke, but these efforts have been in vain.